1. Field of the Invention
The present invention relates to an electric machine having at least two pairs of carbon brushes, which are made up of one plus- and one minus-carbon brush, for the supply of current to the rotor of the machine.
2. Description of Related Art
In the known so-called commutator machines the windings of the rotor are supplied with current via a commutator with the aid of carbon brushes disposed in a stationary manner. Depending on the use of the electric machine, the carbon brushes are subject to varying degrees of wear on their running surface on the commutator during operation. Direct-current motors, which are used for starting internal combustion engines, are exposed to particularly high loads. In most cases the carbon brushes consist of a sintered material predominantly having copper and graphite components. The starter motors are typically designed for short-term operation with a service life of approximately 30,000 to 60,000 switching cycles.
When using starter motors in motor vehicles having an automatic start-stop control, however, considerably more switching cycles are required on account of the frequent start-stop operations. In this context it is important that all carbon brushes of the machine are subjected to loading that is of equal magnitude and uniform nature, if possible. In the currently used starter motors, the carbon brushes are disposed along the periphery of the commutator at an equidistant angular distance, which amounts to 60° in the case of a six-pole machine. In order to reduce the speed ripple, a lamellae number that is not evenly divisible by the pole number is selected for the commutator of the starter motor. For a six-pole machine, for example, commutators having 23 or 28 lamellae are used. This produces different loading of the plus and minus brushes because the brush pairs assume different positions as a function of the direction of rotation when running onto and off the lamellae. This difference results in a different loading profile for the pairs of brushes or for individual carbon brushes, in that certain carbon brushes are acted upon by different current loads compared to the other carbon brush of the brush pair when running up a lamella or down a lamella, depending on the direction of rotation. In addition, considerably higher load peaks are created at the individual carbon brushes. As a result, the carbon brushes age or wear to different degrees, so that the achievable service life of the machine is reduced in a disadvantageous manner due to the greater wear of individual carbon brushes. The carbon brushes of the individual pairs are offset by 180° in each case.
To increase the service life of the electric machine, the lowest and most uniform wear possible is endeavored on all carbon brushes by an optimal distribution of the current load and the load peaks at the carbon brushes. In this context it has already been proposed to set the plus and minus brush across from each other at an angle that deviates from 180° for at least one pair of brushes of the machine. Such an asymmetrical placement of the carbon brushes makes it possible to achieve an optimal distribution of the electric load and the load peaks at the carbon brushes by the lamellae span of the carbon brushes, which generally amounts to one to one and a half lamella(e). Since, depending on the running direction of the commutator, the offset of individual carbon brushes from the symmetrical position differs for an optimal commutation, a counterclockwise rotation requires a different placement than a clockwise rotation.
In addition, placements of individual carbon brushes featuring varying degrees of asymmetry result as a function of the number of lamellae. Since the carbon brushes are accommodated in brush holders, which are inserted on a fixedly mounted brush plate via a holding element in the known manner, it is disadvantageous if a multitude of different brush plates and brush holders is required in order to achieve the necessary asymmetrical placement of individual carbon brushes.
An object of the present invention is to keep the number of parts for obtaining an asymmetrical placement of individual carbon brushes to a minimum.